The invention relates to a magnetic resonance method for forming a fast dynamic imaging form a plurality of signals acquired by an array of multiple sensors. The invention further relates to a magnetic resonance imaging apparatus for obtaining a fast dynamic image and to a computer program product according to the disclosed method.
In magnetic resonance imaging there is a general tendency to obtain acceptable images in shorter time. For this reason there have been developed recently the sensitivity encoding method called xe2x80x9cSENSExe2x80x9d by the Institute of Biomedical Engineering and Medical Informations, University and ETH Zurich, Switzerland. The SENSE method is based on an algorithm which directly acts on the image as detected by the coils of the magnetic resonance apparatus, wherein subsequent encoding steps can be skipped and thus an acceleration of the signal acquisition for imaging of a factor two to three can be obtained. Crucial for the SENSE method is the knowledge of the sensitivity of the coils, which are arrange in so called sensitivity maps. In order to accelerate this method there are proposals to use raw sensitivity maps which can be obtained through division by either the xe2x80x9csum-of-squaresxe2x80x9d of the single coil references or by an optional body coil reference (see e.g. K. Pruessmann et. al. in Proc. ISMRM, 1998, abstracts p. 579, 799, 803 and 2087.
The SENSE method is preferred for acceleration of the signal acquisition for magnetic resonance imaging, which results in an enormous reduction in operating time. However, the method can only be used properly if the coil sensitivity is exactly known. Otherwise imperfections will cause fold-over artefacts which provide wrong images. In practice the coil sensitivity cannot be estimated perfectly and will be dependent on fluctuations in time (movement of the patient, temperature influences, etc.).
Another important problem of the SENSE method is the spatially varying noise level in the resulting image. More specifically, the resulting image can have regions of extremely high noise level, due to local xe2x80x9cunderdeterminationxe2x80x9d of the information provided by the coil patterns.
It is an object of the present invention to reduce the noise level over the full image during fast dynamic imaging to a great extent.
The main issue of the present invention is based on the solution to acquire the low-spatial frequencies, i.e. the low-order k-lines, with full density by using the normal magnetic resonance imaging, and to acquire the remaining high-spatial frequencies, i.e. the high-order k-lines, with reduced density, i.e. with the SENSE imaging method. In more general terms: it is proposed to have a k-space coverage that is varying over k-space, the density of the acquired information being higher in the centre than for the high-spacial frequency information.
An important additional advantage of the imaging in full density of the centre of k-space is that the SENSE acquisition can be reconstructed without a-priori knowledge of coil sensitivity maps. Thus, more accurate images with an improved signal-to-noise ratio can be made at the same rate as with the SENSE method.
These and other advantages of the invention are disclosed in the dependent claims and in the following description in which an exemplified embodiment of the invention is described with respect to the accompanying drawings. It shows